1. Field of the Invention
This invention relates to an antenna coupler for vehicles and more particularly to a coupler mounted on insulator without damaging it for transmitting high frequency signals.
2. Prior Art
For transmitting high frequency signals through an insulator such as glass, etc. it is preferable to install a signal transmitting device which does not damage the insulator. For instance, when connecting communication equipment in an automobile to an antenna provided outside of the automobile, it is desirable to provide the antenna without damaging the automobile body or the window glass thereof.
An inductance coupling type transmission channel coupler for antennas is one which meet the above mentioned requirement. In this coupler, glass is sandwiched by a pair of loop coils such that electromagnetic coupling is made between the two loop coils. The advantage of this device is that the transmission loss is relatively low and the frequency characteristics are even.
In order to obtain an antenna having good antenna characteristics, it is necessary to increase the coupling strength the inductive coupling of the transmission channel coupler. On the other hand, the coupler must be small in size so that it is usable for vehicles, otherwise it cannot meet such usage. However, if the coupler is designed small, the coupling coefficiency of the inductive coupling becomes too small to obtain the predesignated antenna performance.